1. Field of the Invention
The present invention generally relates to an apparatus that calibrates thermometers measuring low temperatures and, particularly, relates to a temperature variable low-temperature calibrating apparatus for thermometers capable of carrying out calibration within the temperature range of 90 K to 170 K, or as low as 77 K.
2. Description of the Related Art
Sheath-type thermometers have a temperature sensor at a tip thereof and are several tens of centimeters in length. Calibration of the sheath-like thermometer for low temperatures is performed as follows. Platinum resistance thermometers that meet conditions for the International Temperature Scale (the current temperature standard is the International Temperature Scale of 1990, documented in 1990) are calibrated on the basis of realized temperature fixed points for the International Temperature Scale, such as the water triple point of 273.16 K, the mercury triple point of 234.3156 K, and the argon triple point of 83.8058 K. For any temperature between these temperature fixed points, temperature scale of the platinum resistance thermometers is obtained by interpolation performed using calibrated values based on the temperature fixed points, in accordance with a method specified for the International Temperature Scale.
However, some thermometers, such as industrial sheath-like thermometers, cannot be calibrated using the temperature fixed points of the International Temperature Scale. Thus, for these thermometers, calibration is made by comparing a calibration target thermometer 210 with a thermometer 200 that meets the conditions for the International Temperature Scale and has been calibrated in accordance with the method specified for the International Temperature Scale hereinafter referred to as a “reference thermometer”) by immersing the thermometers 200, 210 in a cold bath 220A maintained at a temperature (the calibration based on comparison is hereinafter referred to as “comparative calibration”).
For example, in the related art comparative calibration shown in FIG. 3A, calibration for any temperature is performed in a cold bath 220A using a cooling liquid such as alcohol or silicone oil. However, the cooling liquid 220A has its viscosity increased or is solidified at low temperatures, which limits the lower limit on available temperatures. For currently commercially available comparative calibration apparatuses, this lower limit is about 170 K.
Accordingly, in order to perform calibration for temperatures lower than 170 K, a cold bath is employed which uses liquefied gas as a freezing medium 220B, as shown in FIG. 3B. However, for open air cold baths using, for example, liquefied oxygen or nitrogen, the calibration temperature is limited by the properties of the freezing medium, for example, the boiling point of oxygen (90.197 K) or nitrogen (77.352 K). Calibration apparatuses that vary temperature depending on the pressure of liquefied gas (for example, measured by a pressure gauge 230) have also been proposed. However, for these apparatuses, the range of temperature variations is limited by the physical properties of the freezing medium. Further, the pressure of the liquefied gas needs to be increased to raise the temperature, which is disadvantageous in terms of safety.
Consequently, would be desirable to provide an apparatus for calibration for the temperature range of 90 K to 170 K, or even as low as 77 K, that does not use any cooling liquid or liquid freezing medium and which is thus easy to operate.
Further, the related art calibration apparatuses require the reference thermometer 200 to be removed from the apparatus before and after comparative calibration and to be mounted in an apparatus that provides temperature fixed points for the International Temperature Scale in order to check the thermometer for stability. However, vibration that may occur during the mounting or removal is a major cause of serious adverse effects on the performance of the reference thermometer. Therefore, it would be desirable to provide a calibration apparatus that allows the reference thermometer's performance to be checked for its performance during calibration without the need to remove the reference thermometer from the calibration apparatus.
Some related art calibration apparatuses are described in, for example, Japanese Patent No. 3465402; Japanese Patent Laid-Open No. 2004-317193; Japanese Patent No. 2990276; P. Bloembergen, G. Bonnier and H. Ronsin, “An International Intercomparison of Argon Triple Point Calibration Facilities, Accommodating Long-stem Thermometers” Metrologia 27 (1990) pp. 101-106; G. Furukawa, “Argon triple point apparatus with multiple thermometer wells” in Temperature: Its Measurement and Control in Science and Industry, Vol. 6, Part 1, American Institute of Physics, (1992) pp. 265-299; and S. L. Pond, “Argon Triple-Point Apparatus for SPRT Calibration”, in Temperature: Its Measurement and Control in Science and Industry, Vol. 7, Part 1, American Institute of Physics, (2002) pp. 203-208.
As described above, the related art apparatuses for comparative calibration of a sheath-like thermometer use a cooling liquid such as alcohol or silicone oil and a low-temperature freezing medium such as liquefied nitrogen. However, this technique disadvantageously limits the calibration temperature to the one determined by the properties of the freezing medium used.
Further, the related art comparative calibration apparatuses require the reference thermometer to be removed from the apparatus before and after comparative calibration and then mounted in an apparatus for realizing temperature fixed points for the International Temperature Scale in order to check the thermometer for stability. Vibration may occur during the mounting or removal, which is a major cause of serious adverse effects on the performance of the reference thermometer.